Generally, it is easy to obtain polystyrene beads having a high expansion ratio. The resulting foamed articles made from such beads have high rigidity and good shape retention, but have the disadvantage in that they are fragile and have poor chemical resistance, oil resistance and thermal stability. In pre-expanded bead form, polystyrene has a glass transition temperature of, for example, 80.degree.-95.degree. C., precluding its use in automotive foams, for example, under the hood.
Foamed products of polystyrene and styrene-maleic anhydride are known, e.g., from U.S. Pat. No. 4,442,232 and, although they have higher thermal resistance, they are rather difficult to prepare, and have limitations in their impact resistance and compressive strength. Expandable thermoplastic resin beads comprising ethylene-propylene copolymers grafted with vinyl aromatic monomers are also known, e.g., from Kajimura et al., U.S. Pat. No. 4,303,756. The compositions which are produced are said to have excellent thermal stability, but resistance to solvents and oxidation tend to be lower than desirable. To overcome these drawbacks, it has also been proposed to form foams from blends of polyphenylene ethers and polystyrene or high impact, i.e., rubber-modified grafted polystyrene imbibed with liquid blowing agents. Mention can be made of U.S. Pat. No. 3,492,249, which suggests foaming a physical blend of polyphenylene ether and polystyrene. However, to develop maximum strength, the cells have to be elongated and involves a hot-stretching step, which is not desirable. In U.S. Pat. Nos. 4,598,100 and 4,598,101, blends of a polyphenylene ether resin and high impact polystyrene are imbibed at atmospheric pressure with a volatile chlorinated hydrocarbon in an extruder, and the blend is extruded thereafter into a foam; U.S. Pat. Nos. 4,532,263 and 4,598,104, disclose impregnating pellets of a blend of polyphenylene ether and high impact polystyrene, preexpanding the pellets, and then shaping them in an open mold to form foam. The last-mentioned patent also discloses that foamed sheets of blended polyphenylene ether resin an polystyrene can be thermoformed into shaped foamed articles. In all cases, the prior art compositions of the polyphenylene ether resin do not provide ultimate resistance to thermal and physical shocks because they either contain rubber modified polystyrene and they always are merely physical blends of the polymer components. In all cases, the expanded pellets are not of optimum size for molding.
A new method of making expandable particles and foams from polyphenylene ethers has now been discovered. This involves interpolymerization of a vinyl aromatic monomer with a preformed polyphenylene ether resin with the object to make a low density higher temperature resistant expandable bead foam, the beads having a smaller size and more spherical shape than previously obtainable. When, for example, foamable particles are made from a polymerized vinyl monomer, e.g., styrene, containing polyphenylene ether resin, e.g., poly(2,6-dimethyl-1,4-phenylene) ether, the particulate product comprises an interpolymer and thus is different in this respect from the pelletized blends employed in the prior art. The interpolymer has the significant and unexpected ability to increase the glass transition temperature of the preexpanded beads to 104.degree.-117.degree. C., typically, and the high temperature stability of the ultimate foams can be increased to or higher than a previously attainable degree merely by increasing the polyphenylene ether content of the small, spherical particles of interpolymer used to make the foam. Other physical properties are improved, as well, especially, uniform cell structure and foam strength.